Method and apparatus for manufacturing polymer fiber shells via electrospinning

ABSTRACT

An apparatus for manufacturing a polymer fiber shell from liquefied polymer is provided. The apparatus includes: (a) a precipitation electrode being for generating the polymer fiber shell thereupon; (b) a dispenser, being at a first potential relative to the precipitation electrode so as to generate an electric field between the precipitation electrode and the dispenser, the dispenser being for: (i) charging the liquefied polymer thereby providing a charged liquefied polymer; and (ii) dispensing the charged liquefied polymer in a direction of the precipitation electrode; and (c) a subsidiary electrode being at a second potential relative to the precipitation electrode, the subsidiary electrode being for modifying the electric field between the precipitation electrode and the dispenser.

FIELD AND BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method and apparatus formanufacturing polymer fiber shells via electrospinning.

[0002] Polymer fiber shells such as tubular shaped products, are used inthe medical industry for various utilities including esophageal grafts,vascular grafts, stent coats and like.

[0003] Numerous methods for manufacturing polymer fiber shells suitablefor medical applications are known in the art, including, for example,various injection molding methods, mandrel assisted extrusion orformation and various weaving techniques.

[0004] Production of polymer fiber shells suitable for use as vasculargrafts is particularly difficult, since such grafts must withstand highand pulsatile blood pressures while, at the same time, be elastic andbiocompatible.

[0005] Vascular grafts known in the art typically have a microporousstructure that in general allows tissue growth and cell endothelization,thus contributing to long term engraftment and patency of the graft.

[0006] In vascular grafts, tissue ingrowth and cell endothelization istypically enhanced with increased in grafts exhibiting increasedporosity. However, increasing the porosity of vascular grafts leads to aconsiderable reduction of the mechanical and tensile strength of thegraft, and as a consequence to a reduction in the functionality thereof.

[0007] Electrospinning has been used for generating various products formedical applications, e.g., wound dressings, prosthetic devices, andvascular grafts as well as for industrial use, e.g., electrolytic celldiaphragms, battery separators, and fuel cell components. It has alreadybeen proposed to produce by electrospinning products having theappearance of shells. For example, U.S. Pat. No. 4,323,525 discloses amethod of preparing a tubular product by electrostatically spinning afiber forming material and collecting the resulting spun fibers on arotating mandrel. U.S. Pat. No. 4,552,707 discloses a varying rotationrate mandrel which controls the “anisotropy extent” of fiber orientationof the final product. Additional examples of tubular shaped products anda like are disclosed, e.g., in U.S. Pat. Nos. 4,043,331, 4,127,706,4,143,196, 4,223,101, 4,230,650 and 4,345,414.

[0008] The process of electrospinning creates a fine stream or jet ofliquid that upon proper evaporation yields a non-woven fiber structure.The fine stream of liquid is produced by pulling a small amount of aliquefied polymer (either polymer dissolved in solvent (polymersolution) or melted polymer) through space using electrical forces. Theproduced fibers are then collected on a suitably located precipitationdevice, such as a mandrel to form tubular structures. In the case of amelted polymer which is normally solid at room temperature, thehardening procedure may be mere cooling, however other procedures suchas chemical hardening or evaporation of solvent may also be employed.

[0009] In electrospinning, an electric field with high filed linesdensity (i.e., having large magnitude per unit volume) may results in acorona discharge near the precipitation device, and consequently preventfibers from being collected by the precipitation device. The filed linesdensity of an electric field is determined inter alia by the geometry ofthe precipitation device; in particular, sharp edges on theprecipitation device increase the effect of corona discharge.

[0010] In addition, due to the effect of electric dipole rotation alongthe electric field maximal strength vector in the vicinity of themandrel, products with at least a section with a small radius ofcurvature are coated coaxially by the fibers. Such structural fiberformation considerably reduces the radial tensile strength of a spunproduct, which, in the case of vascular grafts, is necessary forwithstanding pressures generated by blood flow.

[0011] Various electrospinning based manufacturing methods forgenerating vascular grafts are known in the prior art, see, for example,U.S. Pat. Nos. 4,044,404, 4,323,525, 4,738,740, 4,743,252, and5,575,818. However, such methods suffer from the above inherentlimitations which limit the use thereof when generating intricateprofile fiber shells.

[0012] Hence, although electrospinning can be efficiently used forgenerating large diameter shells, the nature of the electrospinningprocess prevents efficient generation of products having an intricateprofile and/or small diameter, such as vascular grafts. In particular,since porosity and radial strength are conflicting, prior artelectrospinning methods cannot be effectively used for manufacturingvascular grafts having both characteristics.

[0013] There is thus a widely recognized need for, and it would behighly advantageous to have, a method and apparatus for manufacturingpolymer fiber shells via electrospinning devoid of the abovelimitations.

SUMMARY OF THE INVENTION

[0014] According to one aspect of the present invention there isprovided an apparatus for manufacturing polymer fiber shells fromliquefied polymer, the apparatus comprising: (a) a precipitationelectrode being for generating the polymer fiber shell thereupon; (b) adispenser, being at a first potential relative to the precipitationelectrode so as to generate an electric field between the precipitationelectrode and the dispenser, the dispenser being for: (i) charging theliquefied polymer thereby providing a charged liquefied polymer; and(ii) dispensing the charged liquefied polymer in a direction of theprecipitation electrode; and (c) a subsidiary electrode being at asecond potential relative to the precipitation electrode, the subsidiaryelectrode being for modifying the electric field between theprecipitation electrode and the dispenser.

[0015] According to another aspect of the present invention there isprovided a method for forming a liquefied polymer into a non-wovenpolymer fiber shells, the method comprising: (a) charging the liquefiedpolymer thereby producing a charged liquefied polymer; (b) subjectingthe charged liquefied polymer to a first electric field; (c) dispensingthe charged liquefied polymer within the first electric field in adirection of a precipitation electrode, the precipitation electrodebeing designed and configured for generating the polymer fiber shell;(d) providing a second electric field being for modifying the firstelectric field; and (e) using the precipitation electrode to collect thecharged liquefied polymer thereupon, thereby forming the non-wovenpolymer fiber shell.

[0016] According to further features in preferred embodiments of theinvention described below, the first electric field is defined betweenthe precipitation electrode and a dispensing electrode being at a firstpotential relative to the precipitation electrode.

[0017] According to still further features in the described preferredembodiments step (c) is effected by dispensing the charged liquefiedpolymer from the dispensing electrode.

[0018] According to still further features in the described preferredembodiments the second electric field is defined by a subsidiaryelectrode being at a second potential relative to the precipitationelectrode.

[0019] According to still further features in the described preferredembodiments the subsidiary electrode serves for reducingnon-uniformities in the first electric field

[0020] According to still further features in the described preferredembodiments the subsidiary electrode serves for controlling fiberorientation of the polymer fiber shell generated upon the precipitationelectrode.

[0021] According to still further features in the described preferredembodiments the subsidiary electrode serves to minimize a volume chargegenerated between the dispenser and the precipitation electrode.

[0022] According to still further features in the described preferredembodiments the method further comprising moving the subsidiaryelectrode along the precipitation electrode during step (e).

[0023] According to still further features in the described preferredembodiments the method further comprising moving the dispensingelectrode along the precipitation electrode during step (c).

[0024] According to still further features in the described preferredembodiments the method further comprising synchronizing the motion ofthe dispensing electrode and the subsidiary electrode along theprecipitation electrode.

[0025] According to still further features in the described preferredembodiments the dispenser comprises a mechanism for forming a jet of thecharged liquefied polymer.

[0026] According to still further features in the described preferredembodiments the apparatus further comprising a bath for holding theliquefied polymer.

[0027] According to still further features in the described preferredembodiments the mechanism for forming a jet of the charged liquefiedpolymer includes a dispensing electrode.

[0028] According to still further features in the described preferredembodiments the dispenser is operative to move along a length of theprecipitation electrode.

[0029] According to still further features in the described preferredembodiments the precipitation electrode includes at least one rotatingmandrel.

[0030] According to still further features in the described preferredembodiments the rotating mandrel is a cylindrical mandrel.

[0031] According to still further features in the described preferredembodiments the rotating mandrel is an intricate-profile mandrel.

[0032] According to still further features in the described preferredembodiments the intricate-profile mandrel includes sharp structuralelements.

[0033] According to still further features in the described preferredembodiments the cylindrical mandrel is of a diameter selected from arange of 0.1 to 20 millimeters.

[0034] According to still further features in the described preferredembodiments the precipitation electrode includes at least one structuralelement selected from the group consisting of a protrusion, an orifice,a groove, and a grind.

[0035] According to still further features in the described preferredembodiments the subsidiary electrode is of a shape selected from thegroup consisting of a plane, a cylinder, a torus and a wire.

[0036] According to still further features in the described preferredembodiments the subsidiary electrode is operative to move along a lengthof the precipitation electrode.

[0037] According to still further features in the described preferredembodiments the subsidiary electrode is tilted at angle with respect toa longitudinal axis of the precipitation electrode, the angle is rangingbetween 45 and 90 degrees.

[0038] According to still further features in the described preferredembodiments the subsidiary electrode is positioned at a distance of 5-70millimeters from the precipitation electrode.

[0039] According to still further features in the described preferredembodiments the subsidiary electrode is positioned at a distance δ fromthe precipitation electrode, δ being equal to 12βR(1−V₂/V₁), where β isa constant ranging between about 0.7 and about 0.9, R is thecurvature-radius of the polymer fiber shell formed on the precipitationelectrode, V₁ is the first potential and V₂ is the second potential.

[0040] According to yet another aspect of the present invention there isprovided an apparatus for manufacturing a polymer fiber shells fromliquefied polymer, the apparatus comprising: (a) a dispenser, for: (i)charging the liquefied polymer thereby providing a charged liquefiedpolymer; and (ii) dispensing the charged liquefied polymer; and (b) aprecipitation electrode being at a potential relative to the dispenserthereby generating an electric field between the precipitation electrodeand the dispenser, the precipitation electrode being for collecting thecharged liquefied polymer drawn by the electric field, to thereby formthe polymer fiber shell thereupon, wherein the precipitation electrodeis designed so as to reduce non-uniformities in the electric field.

[0041] According to still further features in the described preferredembodiments the precipitation electrode is formed from a combination ofelectroconductive and non-electroconductive materials.

[0042] According to still further features in the described preferredembodiments a surface of the precipitation electrode is formed by apredetermined pattern of the electroconductive and non-electroconductivematerials.

[0043] According to still further features in the described preferredembodiments the precipitation electrode is formed from at least twolayers.

[0044] According to still further features in the described preferredembodiments the at least two layers include an electroconductive layerand a partial electroconductive layer.

[0045] According to still further features in the described preferredembodiments the partial electroconductive layer is partialelectroconductive layer is formed from a combination of anelectroconductive material and at least one dielectric material.

[0046] According to still further features in the described preferredembodiments the dielectric material is selected from a group consistingof polyamide and polyacrylonitrile and polytetrafluoroethylene.

[0047] According to still further features in the described preferredembodiments the dielectric material is Titanium Nitride.

[0048] According to still further features in the described preferredembodiments the partial electroconductive layer, is selected of athickness ranging between 0.1 to 90 microns.

[0049] The present invention successfully addresses the shortcomings ofthe presently known configurations by providing an electrospinningapparatus and method capable of fabricating a non-woven polymer fibershell which can be used in vascular grafts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

[0051] In the drawings:

[0052]FIG. 1 is a schematic illustration of a prior art electrospinningapparatus;

[0053]FIG. 2 is a schematic illustration of an electrospinning apparatuswhich includes a subsidiary electrode according to the teachings of thepresent invention;

[0054]FIG. 3 is a schematic illustration of an electrospinning apparatuswhich includes a planar subsidiary electrode according to the teachingsof the present invention;

[0055]FIG. 4 is a schematic illustration of an electrospinning apparatuswhich includes a cylindrical subsidiary electrode according to theteachings of the present invention;

[0056]FIG. 5 is a schematic illustration of an electrospinning apparatuswhich includes a linear subsidiary electrode according to the teachingsof the present invention;

[0057]FIG. 6 is a schematic illustration of an electrospinning apparatuswhich includes a composite subsidiary electrode according to theteachings of the present invention;

[0058]FIG. 7 is an electron microscope image of material spun usingconventional electrospinning techniques;

[0059]FIG. 8 is an electron microscope image of material spun using anapparatus which incorporates a flat subsidiary electrode, positioned 20millimeters from the mandrel, according to the teachings of the presentinvention;

[0060]FIG. 9 is an electron microscope image of material spun using anapparatus which incorporates a flat subsidiary electrode, positioned 9millimeters from the mandrel, according to the teachings of the presentinvention; and

[0061]FIG. 10 is an electron microscope image of polar-oriented materialspun using an apparatus which incorporates a linear subsidiary electrodeaccording to the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] The present invention is of a method and an apparatus formanufacturing a polymer fiber shell using electrospinning. Specifically,the present invention can be used to manufacture intricate-profileproducts and vascular grafts of small to large diameter viaelectrospinning.

[0063] For purposes of better understanding the present invention, asillustrated in FIGS. 2-10 of the drawings, reference is first made tothe construction and operation of a conventional (i.e., prior art)electrospinning apparatus as illustrated in FIG. 1.

[0064]FIG. 1 illustrates an apparatus for manufacturing a tubularstructure using a conventional electrospinning apparatus, which isreferred to herein as apparatus 10.

[0065] Apparatus 10 includes a dispenser 12 which can be, for example, abath provided with capillary apertures 14. Dispenser 12 serves forstoring the polymer to be spun in a liquid form. Dispenser 12 ispositioned at a predetermined distance from a precipitation electrode16.

[0066] Precipitation electrode 16 serves for generating the tubularstructure thereupon. Precipitation electrode 16 is typicallymanufactured in the form of a mandrel or any other cylindricalstructure. Precipitation electrode 16 is rotated by a mechanism suchthat a tubular structure is formed when coated with the polymer.

[0067] Dispenser 12 is typically grounded, while precipitation electrode16 is connected to a source of high voltage preferably of negativepolarity, thus forming an electric field between dispenser 12 andprecipitation electrode 16. Alternatively, precipitation electrode 16can be grounded while dispenser 12 is connected to a source of highvoltage, preferably with positive polarity.

[0068] To generate a tubular structure, a liquefied polymer (e.g.,melted polymer or dissolved polymer) is extruded, for example under theaction of hydrostatic pressure, through capillary apertures 14 ofdispenser 12. As soon as meniscus forms from the extruded liquefiedpolymer, a process of solvent evaporation or cooling starts which isaccompanied by the creation of capsules with a semi-rigid envelope orcrust. An electric field, occasionally accompanied a by unipolar coronadischarge in the area of dispenser 12, is generated by the potentialdifference between dispenser 12 and precipitation electrode 16. Becausethe liquefied polymer possesses a certain degree of electricalconductivity, the above-described capsules become charged. Electricforces of repulsion within the capsules lead to a drastic increase inhydrostatic pressure. The semi-rigid envelopes are stretched, and anumber of point micro-ruptures are formed on the surface of eachenvelope leading to spraying of ultra-thin jets of liquefied polymerfrom dispenser 12.

[0069] The charges tend to distribute along the jets, thus preventingexistence of any non-zero component of electric field inside the jet.Thus, a conduction current flows along the jets, which results in theaccumulation of (different sign) free charges on the liquefied polymersurface.

[0070] Under the effect of a Coulomb force, the jets depart from thedispenser 12 and travel towards the opposite polarity electrode, i.e.,precipitation electrode 16. Moving with high velocity in theinter-electrode space, the jet cools or solvent therein evaporates, thusforming fibers which are collected on the surface of precipitationelectrode 16. Since electrode 16 is rotating the charged fibers form atubular shape.

[0071] When using mandrels being at least partially with small radius ofcurvature, the orientation of the electric field maximal strength vectoris such that precipitation electrode 16 is coated coaxially by thefibers. Thus, small diameter products, have limited radial strength whenmanufactured via existing electrospinning methods, as described above.

[0072] When using mandrels with sharp edges and/or variously shaped andsized recesses, the electric field magnitude in the vicinity ofprecipitation electrode 16 may exceed the air electric strength (about30 kV/cm), and a corona discharge may develop in the area ofprecipitation electrode 16. The effect of corona discharge decreases thecoating efficiency of the process as described hereinbelow, and may evenresultant in a total inability of fibers to be collected uponprecipitation electrode 16.

[0073] Corona discharge initiation is accompanied by the generation of aconsiderable amount of air ions having opposite charge sign with respectto the charged fibers. Since an electric force is directed with respectto the polarity of charges on which it acts, theses ions start to moveat the opposite direction to fibers motion i.e., from precipitationelectrode 16 towards dispenser 12. Consequently, a portion of these ionsgenerate a volume charge (ion cloud), non-uniformly distributed in theinter-electrode space, thereby causing electric field lines to partiallyclose on the volume charge rather than on precipitation electrode 16.Moreover, the existence of an opposite volume charge in theinter-electrode space, decreases the electric force on the fibers, thusresulting in a large amount of fibers accumulating in theinter-electrode space and gradually settling under gravity force. Itwill be appreciated that such an effect leads to a low-efficiencyprocess of fiber coating.

[0074] Using an infinite-length/radius cylinder as a precipitationelectrode 16 diminishes the effect described above. However, this effectis severe and limiting when small radii or complicated mandrels areemployed for fabricating small radius or intricate-profile structures.

[0075] Before explaining at least one embodiment of the invention indetail, it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth in the following description or illustrated in thedrawings. The invention is capable of other embodiments or of beingpracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposeof description and should not be regarded as limiting.

[0076] While reducing the present invention to practice, it wasuncovered that the use of a third electrode within an electrospinningapparatus enables to control the electric field generated between thedispenser and precipitation electrode. Specifically, a third electrodemay either substantially decreases non-uniformities in the electricfield or provides for controlled fiber orientation upon deposition.

[0077] Thus, according to the present invention there is provided anapparatus for manufacturing a polymer fiber shell from a liquefiedpolymer, which apparatus is referred to herein as apparatus 20.

[0078] As shown in FIG. 2, apparatus 20 includes a precipitationelectrode 22 which serves for generating the polymer fiber shellthereupon. Precipitation electrode 22 can be, for example, a mandrel ofuniform or varying radius, which may include some structural elementssuch as, but not limited to, protrusions, orifices and grooves. Thesurface of precipitation electrode 22 may also contain grinds. Thediameter of the mandrel may vary from about 0.1 millimeter up to about20 millimeters depending on the diameter of the polymer fiber shell tobe spun thereupon.

[0079] Apparatus 20 further includes a dispenser 24, which is at a firstpotential relative to precipitation electrode 22. Such a potential canbe generated by grounding dispenser 24, and connecting a source of highvoltage with negative polarity to precipitation electrode 22.

[0080] Alternatively, precipitation electrode 22 can be grounded whiledispenser 24 is connected to a source of high voltage with positivepolarity. In any case, an absolute value for the potential differencebetween dispenser 24 and precipitation electrode 22 may range betweenabout 10 kV and about 100 kV.

[0081] The potential difference between dispenser 24 and precipitationelectrode 22 ensures that an electric field is maintained therebetween,which electric field is important for the electrospinning process asdescribed hereinabove.

[0082] Dispenser 24 serves for charging the liquefied polymer, therebyproviding a charged liquefied polymer and dispensing the chargedliquefied polymer in a direction of precipitation electrode 22.Dispenser 24 may also include a mechanism for moving it along alongitudinal axis of precipitation electrode 22, thus enablingdispensing of the charged liquefied polymer at various points along thelongitudinal axis of precipitation electrode 22.

[0083] The charged liquefied polymer may be, for example polyurethane,polyester, polyolefin, polymethyl methacrylate, polyvinyl aromatic,polyvinyl ester, polyamide, polyimide, polyether, polycarbonate,polyacrilonitrile, polyvinyl pyrrolidone, polyethylene oxide, poly(L-lactic acid), poly (lactide-CD-glycoside), polycaprolactone,polyphosphate ester, poly (glycolic acid), poly (DL-lactic acid), andsome copolymers. Biolmolecules such as DNA, silk, chitozan and cellulosemay also be used. Improved charging of the polymer may also be required.Improved charging is effected according to the present invention bymixing the liquefied polymer with a charge control agent (e.g., adipolar additive) to form, for example, a polymer-dipolar additivecomplex which apparently better interacts with ionized air moleculesformed under the influence of the electric field. It is assumed, in anon-limiting fashion, that the extra-charge attributed to the newlyformed fibers is responsible for their more homogenous precipitation onthe precipitation electrode, wherein a fiber is better attracted to alocal maximum, which is a local position most under represented by olderprecipitated fibers, which keep their charge for 5-10 minutes. Thecharge control agent is typically added in the grams equivalent perliter range, say, in the range of from about 0.001 N to about 0.1 N,depending on the respective molecular weights of the polymer and thecharge control agent used.

[0084] U.S. Pat. Nos. 5,726,107; 5,554,722; and 5,558,809 teach the useof charge control agents in combination with polycondensation processesin the production of electret fibers, which are fibers characterized ina permanent electric charge, using melt spinning and other processesdevoid of the use of an precipitation electrode. A charge control agentis added in such a way that it is incorporated into the melted orpartially melted fibers and remains incorporated therein to provide thefibers with electrostatic charge which is not dissipating for prolongedtime periods, say months.

[0085] In a preferred embodiment of the present invention, the chargecontrol agent transiently binds to the outer surface of the fibers andtherefore the charge dissipates shortly thereafter (within minutes).This is because polycondensation is not exercised at all such thechemical intereaction between the agent and the polymer is absent, andfurther due to the low concentration of charge control agent employed.The resulting shell is therefore substantially charge free.

[0086] Suitable charge control agents include, but are not limited to,mono- and poly-cyclic radicals that can bind to the polymer moleculevia, for example, —C═C—, ═C—SH— or —CO—NH— groups, including biscationicamides, phenol and uryl sulfide derivatives, metal complex compounds,triphenylmethanes, dimethylmidazole and ethoxytrimethylsians.

[0087] Typically, the charged liquefied polymer is dispensed as a liquidjet, moving at high velocity under electrical forces caused by theelectric field. Thus, dispenser 24 typically includes a bath for holdingthe liquefied polymer and a mechanism for forming a jet, which mechanismmay be, for example, a dispensing electrode.

[0088] Apparatus 20 further includes at least one subsidiary electrode26 which is at a second potential relative to precipitation electrode22. Subsidiary electrode 26 serves for controlling the direction andmagnitude of the electric field between precipitation electrode 22 anddispenser 24 and as such, subsidiary electrode 26 can be used to controlthe orientation of polymer fibers deposited on precipitation electrode22. In some embodiments, subsidiary electrode 26 serves as asupplementary screening electrode. Broadly stated, use of screeningresults in decreasing the coating precipitation factor, which isparticularly important upon mandrels having at least a section of smallradii of curvature.

[0089] The size, shape, position and number of subsidiary electrode 26is selected so as to maximize the coating precipitation factor, whileminimizing the effect of corona discharge in the area of precipitationelectrode 22 and/or so as to provide for controlled fiber orientationupon deposition.

[0090] According to one preferred embodiment of the present invention,subsidiary electrode 26 is positioned 5-70 mm away from precipitationelectrode 22.

[0091] Preferably, such a distance is selected according to thefollowing:

δ12βR(1−V ₂ /V ₁)  (Eq. 1)

[0092] where β is a dimensionless constant named a fiber-chargeaccounting factor, which ranges between about 0.7 and about 0.9, R isthe curvature-radius of precipitation electrode 22, V₁ is the potentialdifference between dispenser 24 and precipitation electrode 22 and V₂ isthe potential difference between subsidiary electrode 26 andprecipitation electrode 22.

[0093] Subsidiary electrode 26 may include a mechanism for moving italong a longitudinal axis of precipitation electrode 22. Such amechanism may be in use when enhanced control over fiber orientation isrequired.

[0094] It will be appreciated that in an apparatus in which bothdispenser 24 and subsidiary electrode 26 are capable of suchlongitudinal motion, such motion may be either independent orsynchronized.

[0095] Subsidiary electrode 26 may also be tilted through an angle of45-90 degrees with respect to the longitudinal axis of precipitationelectrode 22. Such tilting may be used to provide for controlled fiberorientation upon deposition, hence to control the radial strength of themanufactured shell; specifically, large angles result in higher radialstrength.

[0096] In addition to positioning, the shape and size of electrode 26may also determine the quality of the shell formed by apparatus 20.Thus, electrode 26 may be fabricated in a variety of shapes each servinga specific purpose. Electrode shapes which can be used with apparatus 20of the present invention include, but are not limited to, a plane, acylinder, a torus a rod, a knife, an arc or a ring.

[0097] An apparatus 20 which includes a subsidiary electrode 26 of acylindrical (FIG. 4) or a flat shape (FIG. 3) enables manufacturingintricate-profile products being at least partially with small radius ofcurvature, which radius may range between 0.025 millimeters and 5millimeters. As can be seen in FIGS. 8-9 (further described in theExamples section), the coating of such structures is characterized byrandom-oriented (FIG. 8) or even polar-oriented (FIG. 9) fibers, asopposed to an axial coating which is typical for small curvatureproducts manufactured via existing electrospinning methods asdemonstrated in FIG. 7 (further described in the Examples section).

[0098] Preferably, when a surface of large curvature is used assubsidiary electrode 26, as is the case above, the distance betweensubsidiary electrode 26 and precipitation electrode 22 can be determinedas δ/x where x is a factor ranging between 1.8 and 2, and where δ is asdefined by Equation 1 above.

[0099] Thus, positioning and/or shape of electrode 26 determines fiberorientation in the polymer fiber shell formed.

[0100] The ability to control fiber orientation is important whenfabricating vascular grafts in which a high radial strength andelasticity is important. It will be appreciated that a polar orientedstructure can generally be obtained also by wet spinning methods,however in wet spinning methods the fibers are thicker than those usedby electrospinning by at least an order of magnitude.

[0101] Control over fiber orientation is also advantageous whenfabricating composite polymer fiber shells which are manufactured bysequential deposition of several different fiber materials.

[0102] Reference is now made to FIG. 5, which illustrates an apparatus20 which utilizes a linear (e.g., a rod, a knife, an arc or a ring)subsidiary electrode 26.

[0103] The effect of subsidiary electrode 26 of linear shape is based onthe distortion it introduces to the electric field in an area adjacentto precipitation electrode 22. For maximum effect the diameter ofsubsidiary electrode 26 must be considerably smaller than that ofprecipitation electrode 22, yet large enough to avoid generation of asignificant corona discharge. Fiber coating generated by apparatus 20utilizing a linear subsidiary electrode 26 is illustrated by FIG. 10which is further described in the Examples section hereinunder.

[0104] Thus, the present invention provides an electrospinning apparatusin which the electric field is under substantial control, therebyproviding either random or predetermined fibers orientation.

[0105] Although the use of at least one subsidiary electrode ispresently preferred, field non-uniformities can also be at leastpartially overcome by providing a composite precipitation electrode.

[0106] As illustrated in FIG. 6, precipitation electrode 34 of apparatus30 having a dispenser 32 can be designed and configured so as to reducenon-uniformities in the electric field.

[0107] To overcome field non-uniformities, precipitation electrode 34 isfabricated from at least two layers of materials, an inner layer 36 madeof electroconductive material and an outer layer 38 made of a materialhaving high dielectric properties. Such a fabrication design results ina considerable increase of corona discharge threshold thus considerablyreducing corona discharge from precipitation electrode 34.

[0108] Materials suitable for use with outer layer 38 of precipitationelectrode 34, can be ceramic materials e.g., Titanium Nitride, AluminumOxide and the like, or polymer materials e.g., polyamide,polyacrylonitrile, polytetrafluoroethylene and the like. The thicknessof outer layer 38 depends on the dielectric properties of the materialfrom which it is made and can vary from less than one micron, in thecase of, for example, a Titanium Nitride layer, or tens of microns, inthe case of, for example, polytetrafluoroethylene, polyamide orpolyacrylonitrile layer. In addition to diminishing corona dischargethis precipitation electrode configuration enables easier separation offormed structures therefrom. Thus, according to this configuration outerlayer 38 of precipitation electrode 34 can also be configured forfacilitating the removal of the final product from the mandrel.

[0109] Additional objects, advantages, and novel features of the presentinvention will become apparent to one ordinarily skilled in the art uponexamination of the following examples, which are not intended to belimiting. Additionally, each of the various embodiments and aspects ofthe present invention as delineated hereinabove and as claimed in theclaims section below finds experimental support in the followingexamples.

EXAMPLES

[0110] Reference is now made to the following examples, which togetherwith the above descriptions, illustrate the invention in a non limitingfashion.

Electrospinning Material

[0111] A polycarbonate resin grade Caliper 2071 was purchased from DawChemical Co. This Polymer is characterized as having good fiber formingabilities and is convenient for electrospinning. Chloroform was used assolvent in all of the examples described hereinbelow.

Example 1 Axial Covering Using Conventional Electrospinning Method

[0112] Reference is now made to FIG. 7, which is an example ofnon-randomized covering of thin mandrels via conventionalelectrospinning. A 3-mm cylindrical mandrel was covered by polycarbonatefiber using prior art electrospinning approaches. FIG. 7 is an electronmicroscope image of the final product, in which axial fiber orientationis well evident. Due to non-uniformities in the electric field, thefibers, while still in motion in the inter-electrode space, are orientedin conformity with the field configuration, and the obtained tubularstructure exhibits axial orientation of fibers, and as such ischaracterized by axial, as opposed to radial strength.

Example 2 Random Covering Using Flat Subsidiary Electrode

[0113] An apparatus constructed and operative in accordance with theteachings of the present invention incorporating a flat subsidiaryelectrode positioned 20 millimeters from the mandrel and having the samepotential as the mandrel was used to spin a polycarbonate tubularstructure of a 3 mm radius. As is evident from FIG. 8, the presence of asubsidiary electrode randomizes fibers orientation.

Example 3 Polar-Oriented Covering Using Flat Subsidiary Electrode

[0114] An apparatus constructed and operative in accordance with theteachings of the present invention incorporating a flat subsidiaryelectrode positioned 9 millimeters from the mandrel and being at apotential difference of 5 kV from the mandrel was used to spin apolycarbonate tubular structure of a 3 mm radius.

[0115] As illustrated by FIG. 9, reduction of equalizingelectrode-mandrel distance results in polar-oriented covering. Thus, bykeeping subsidiary electrode and mandrel within a relatively smalldistance, while providing a non-zero potential difference therebetween,leads to slow or no fiber charge dissipation and, as a result, theinter-electrode space becomes populated with fiber which are heldstatically in a stretched position, oriented perpendicular to mandrelsymmetry axis. Once stretched, the fibers are gradually coiled aroundthe rotating mandrel, generating a polar-oriented structure.

Example 4 Predefined Oriented Covering Using Linear Subsidiary Electrode

[0116]FIG. 10 illustrates result obtained from an apparatusconfiguration which may be employed in order to obtain a predefinedoriented structural fiber covering.

[0117] An apparatus which includes an elliptical subsidiary electrodeand a dispenser both moving along the longitudinal axis of the mandrelin a reciprocating synchronous movement was used to coat a 3-mmcylindrical mandrel with polycarbonate fiber. The subsidiary electrodehad a large diameter of 120 mm, a small diameter of 117.6 mm and athickness of 1.2 mm. The subsidiary electrode was positioned 15 mm fromthe mandrel, at an 80° tilt with respect to the mandrel symmetry axis.

[0118] It is appreciated that certain features of the invention, whichare, for clarity, described in the context of separate embodiments, mayalso be provided in combination in a single embodiment. Conversely,various features of the invention, which are, for brevity, described inthe context of a single embodiment, may also be provided separately orin any suitable subcombination.

[0119] Although the invention has been described in conjunction withspecific embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

What is claimed is:
 1. An apparatus for manufacturing polymer fibershell from liquefied polymer, the apparatus comprising: (a) aprecipitation electrode being for generating the polymer fiber shellthereupon; (b) a dispenser, being at a first potential relative to saidprecipitation electrode so as to generate an electric field between saidprecipitation electrode and said dispenser, said dispenser being for:(i) charging the liquefied polymer thereby providing a charged liquefiedpolymer; and (ii) dispensing said charged liquefied polymer in adirection of said precipitation electrode; and (c) a subsidiaryelectrode being at a second potential relative to said precipitationelectrode, said subsidiary electrode being for modifying said electricfield between said precipitation electrode and said dispenser.
 2. Theapparatus of claim 1, wherein said subsidiary electrode serves forreducing non-uniformities in said electric field between saidprecipitation electrode and said dispenser.
 3. The apparatus of claim 1,wherein said subsidiary electrode serves for controlling fiberorientation of said polymer fiber shell generated upon saidprecipitation electrode.
 4. The apparatus according to claim 1, whereinsaid dispenser comprises a mechanism for forming a jet of said chargedliquefied polymer.
 5. The apparatus according to claim 1, furthercomprising a bath for holding the liquefied polymer.
 6. The apparatusaccording to claim 4, wherein said mechanism for forming a jet of saidcharged liquefied polymer includes a dispensing electrode.
 7. Theapparatus according to claim 1, wherein said dispenser is operative tomove along a longitudinal axis of said precipitation electrode.
 8. Theapparatus according to claim 1, wherein said precipitation electrodeincludes at least one rotating mandrel.
 9. The apparatus according toclaim 8, wherein said rotating mandrel is a cylindrical mandrel.
 10. Theapparatus according to claim 9, wherein said cylindrical mandrel is of adiameter selected from a range of 0.1 to 20 millimeters.
 11. Theapparatus according to claim 8, wherein said rotating mandrel is anintricate-profile mandrel.
 12. The apparatus according to claim 11,wherein said intricate-profile mandrel includes sharp structuralelements.
 13. The apparatus according to claim 1, wherein saidprecipitation electrode includes at least one structural elementselected from the group consisting of a protrusion, an orifice, agroove, and a grind.
 14. The apparatus according to claim 1, whereinsaid subsidiary electrode is of a shape selected from the groupconsisting of a plane, a cylinder, a torus and a wire.
 15. The apparatusaccording to claim 1, wherein said subsidiary electrode is operative tomove along a longitudinal axis of said precipitation electrode.
 16. Theapparatus according to claim 1, wherein said subsidiary electrode istilted at angle with respect to a longitudinal axis of saidprecipitation electrode, said angle is ranging between 45 and 90degrees.
 17. The apparatus according to claim 1, wherein said subsidiaryelectrode is positioned at a distance of 5-70 millimeters from saidprecipitation electrode.
 18. The apparatus according to claim 1, whereinsaid subsidiary electrode is positioned at a distance δ from saidprecipitation electrode, δ being equal to 12βR(1−V₂/V₁), where β is aconstant ranging between about 0.7 and about 0.9, R is acurvature-radius of the polymer fiber shell formed on said precipitationelectrode, V₁ is said first potential and V₂ is said second potential.19. A method for forming a liquefied polymer into a non-woven polymerfiber shell, the method comprising: (a) charging the liquefied polymerthereby producing a charged liquefied polymer; (b) subjecting saidcharged liquefied polymer to a first electric field; (c) dispensing saidcharged liquefied polymer within said first electric field in adirection of a precipitation electrode, said precipitation electrodebeing designed and configured for generating the polymer fiber shellthereupon; (d) providing a second electric field being for modifyingsaid first electric field; and (e) using said precipitation electrode tocollect said charged liquefied polymer thereupon, thereby forming thenon-woven polymer fiber shells.
 20. The method according to claim 19,wherein said first electric field is defined between said precipitationelectrode and a dispensing electrode being at a first potential relativeto said precipitation electrode.
 21. The method according to claim 20,wherein step (c) is effected by dispensing said charged liquefiedpolymer from said dispensing electrode.
 22. The method according toclaim 19, further comprising moving said dispensing electrode along alongitudinal axis of said precipitation electrode during step (c). 23.The method according to claim 19, wherein said precipitation electrodeincludes at least one rotating mandrel.
 24. The method according toclaim 23, wherein said rotating mandrel is a cylindrical mandrel. 25.The method according to claim 24, wherein said cylindrical mandrel is ofa diameter selected from a range of 0.1 to 20 millimeters.
 26. Themethod according to claim 23, wherein said rotating mandrel is anintricate-profile mandrel.
 27. The method according to claim 26, whereinsaid intricate-profile mandrel includes sharp structural elements. 28.The method according to claim 25, wherein said precipitation electrodeincludes at least one structural element selected from the groupconsisting of a protrusion, an orifice, a groove, and a grind.
 29. Themethod according to claim 20, wherein said second electric field isdefined by a subsidiary electrode being at a second potential relativeto said precipitation electrode.
 30. The method according to claim 29,wherein said subsidiary electrode serves for reducing non-uniformitiesin said first electric field.
 31. The method according to claim 19,wherein said subsidiary electrode serves for controlling fiberorientation of said polymer fiber shell generated upon saidprecipitation electrode.
 32. The method according to claim 29, whereinsaid subsidiary electrode is of a shape selected from the groupconsisting of a plane, a cylinder, a torus and a wire.
 33. The methodaccording to claim 29, further comprising moving said subsidiaryelectrode along said precipitation electrode during step (e).
 34. Themethod according to claim 29, further comprising tilting said subsidiaryelectrode at angle with respect to a longitudinal axis of saidprecipitation electrode, said angle ranging between 45 and 90 degrees.35. The method according to claim 29, wherein said subsidiary electrodeis positioned at a distance of 5-50 millimeters from said precipitationelectrode.
 36. The method according to claim 29, wherein said subsidiaryelectrode is positioned at a distance δ from said precipitationelectrode, δ being equal to 12βR(1−V₂/V₁), where β is a constant rangingbetween about 0.7 and about 0.9, R is a curvature-radius of the polymerfiber shell formed on said precipitation electrode, V₁ is said firstpotential and V₂ is said second potential.
 37. An apparatus formanufacturing a polymer fiber shell from liquefied polymer, theapparatus comprising: (a) a dispenser, for: (i) charging the liquefiedpolymer thereby providing a charged liquefied polymer; and (ii)dispensing said charged liquefied polymer; and (b) a precipitationelectrode being at a potential relative to said dispenser therebygenerating an electric field between said precipitation electrode andsaid dispenser, said precipitation electrode being for collecting saidcharged liquefied polymer drawn by said electric field, to thereby formthe polymer fiber shell thereupon, wherein said precipitation electrodeis designed so as to reduce non-uniformities in said electric field. 38.The apparatus according to claim 37, wherein said dispenser comprises amechanism for forming a jet of said charged liquefied polymer.
 39. Theapparatus according to claim 37, further comprising a bath for holdingthe liquefied polymer.
 40. The apparatus according to claim 38, whereinsaid mechanism for forming a jet of said charged liquefied polymerincludes a dispensing electrode.
 41. The apparatus according to claim37, wherein said precipitation electrode is formed from a combination ofelectroconductive and non-electroconductive materials.
 42. The apparatusaccording to claim 41, wherein a surface of said precipitation electrodeis formed from a predetermined pattern of said electroconductive andnon-electroconductive materials.
 43. The apparatus according to claim37, wherein said precipitation electrode is formed from at least twolayers.
 44. The apparatus according to claim 43, wherein said at leasttwo layers include an electroconductive layer and a partialelectroconductive layer.
 45. The apparatus according to claim 44,wherein said partial electroconductive layer is formed from acombination of an electroconductive material and at least one dielectricmaterial.
 46. The apparatus according to claim 45, wherein saiddielectric material is selected from a group consisting of polyamide,polytetrafluoroethylene and polyacrylonitrile.
 47. The apparatusaccording to claim 45, wherein said dielectric material is TitaniumNitride.
 48. The apparatus according to claim 44, wherein said partiallyelectroconductive layer, is of a thickness selected from a range of 0.1to 90 microns.
 49. The apparatus according to claim 37, wherein saidprecipitation electrode is of a diameter selected from a range of 0.1 to20 millimeters.
 50. The apparatus according to claim 37, wherein saidprecipitation electrode includes at least one rotating mandrel.
 51. Theapparatus according to claim 50, wherein said rotating mandrel is acylindrical mandrel.
 52. A tubular structure manufactured by theapparatus of claim
 1. 53. A tubular structure manufactured by the methodof claim
 29. 54. A tubular structure manufactured by the apparatus ofclaim
 37. 55. The apparatus of claim 1, wherein said subsidiaryelectrode serves to minimize a volume charge generated between saiddispenser and said precipitation electrode.
 56. The method according toclaim 29, wherein said subsidiary electrode serves to minimize a volumecharge generated between said precipitation electrode and saiddispensing electrode.
 57. The apparatus according to claim 1, whereinsaid dispenser and said subsidiary electrode are operative to movesynchronically along a longitudinal axis of said precipitationelectrode.
 58. The method according to claim 29, further comprisingsynchronically moving both said dispensing electrode and said subsidiaryelectrode along said precipitation electrode.